This and the referenced inventions are addressed to the related national problems of energy security and air quality. In particular, it is addressed to potential domestic energy resources which are not utilized, or under utilized, because of impurities, notably moisture and chlorine.
Fuels (with the exception of nuclear fuels) are said to be carbonaceous, i.e., having a carbon skeleton. The fluid fuels: oil and gas, are essentially mixtures of hydrocarbons whereas solid fuels have considerable oxygen in their molecular structure.
Coals are "ranked" according to their geological age. Those of high rank (oldest) have high carbon and low oxygen contents, little affinity for water, and are mineral-like. Anthracite and bituminous coals are considered high rank. As rank (age) decreases coals have decreasing carbon and increasing oxygen contents and affinity for water, and become more fibrous. Sub-bituminous and lignites are low rank coals. Although not called coal, peat is a fossil fuel still lower in age or rank.
Sub-bituminous coals and lignites are important commercial fuels, usually mined at low cost. (Powder River Basin sub-bituminous has another attractive feature--it is low in sulfur and in considerable demand by utilities having difficulty meeting sulfur dioxide emission regulations.) However, their high moisture contents, and correspondingly low heating values, make them expensive to ship to market and inefficient to burn.
Heating values are misleadingly reported as though water were only a diluent. Besides dilution, additional energy is wasted evaporating it, making this impurity an even greater drawback than is apparent, especially for solid fuels shipped at high expense.
Still further down the ranking scale are a variety of organic wastes and by-products (biomass), whose aggregate dry heating value, although not usually counted as an energy resource, could make an appreciable contribution to the domestic supply. Among these are Municipal Solid Waste, Industrial Wastes, Construction and Demolition Wastes, Paper Mill and Sewage Sludges. To these can be added the variety of woody or cellulosic by-products of agriculture and forestry, and industry based upon them. Compared with even lowest ranked fossil fuels, they have lower contents of carbon and higher contents of oxygen. Most are also fibrous and normally associated with substantial water.
Inventors and entrepreneurs have responded to the moisture/heating value drawback of low rank coals by putting forward a variety of carbonization processes, in which moisture and oxygen are driven off by heat. In other words, the carbonaceous raw material has been enriched in carbon. Although heating value is improved, the product tends to be troubled by dusting and spontaneous combustion. By product water, heavily contaminated with complex organic chemicals, presents a difficult disposal problem (addressed in my U.S. Pat. No. 5,000,099). These carbonizations, moreover, handle and process raw material and product as solid fuels, through a sequence of solids moving, heating, cooling, crushing, screening, etc. steps, at considerable expense and opportunity for pollution and loss.
In contrast to the ease, economy and cleanliness with which fluid fuels are pipelined around the country, solids are burdened with open mechanical excavators, conveyors, bulldozers, crushers, hoppers, railroad cars, pilers, reclaimers, grinders, etc. all of which require labor and create noise, dust, loss and polluted runoff. The existing mechanical culture of solid fuels utilization needs lump fuel of limited size range. In crushing oversize material to conform, considerable undersized must be rejected. These "fines" have little market and comprise not only a loss of material but an environmental debit.
The DOE and private entrepreneurs have tried to address the often overlooked form penalty, which solid fuels have to bear relative to fluid fuels: oil and gas. One of the most extensive of such attempts has comprised programs to convert coal into a liquid slurry fuel, called Coal-Water-Fuel (CWF), which has been successfully fired in boilers and furnaces designed for oil. Specially prepared CWFs have also been fired in experimental diesel engines and gas turbine combustors. Unfortunately, most coals require extensive beneficiation and expensive additives, making the cost of energy, in CWF form, roughly double that of the coal from which it is made. At the time of this application world oil prices are so low that this technically feasible substitution is uneconomic.
High rank coals (anthracite and bituminous) can be ground and slurried to a pumpable solids concentration of 50% or higher. As rank decreases (sub-bituminous toward lignite), there is a deterioration in slurryability. Poor slurrying characteristics of low rank and waste fuels are associated with their fibrous and hydrophillic nature. However, it was pointed out in my U.S. Pat. No. 4,380,960 that a slurry of a hydrophillic fuel can be concentrated by heating to a temperature at which molecular rearrangement occurs, with splitting off of carbon dioxide and water, resulting in a less hydrophilic and fibrous fuel (char) for which the maximum pumpable concentration is considerably increased. I have called this process "Slurry Carbonization".
The Energy and Environmental Research Center (EERC) of the University of North Dakota has extensively studied the slurry carbonization (which they call Hydrothermal Treatment or Hot Water Drying) of North Dakota and other lignites. In a continuous pilot unit EERC, under contract with the DOE, has carbonized a slurry of low ash sub bituminous coal which, after concentration, the Allison Division of General Motors used successfully to fuel a commercial-scale gas turbine with solid fuel for the first time.
The EERC has also studied the slurry carbonization of such woody by-products as sawdust, obtaining around 300% improvement in the concentration (and energy density) of pumpable slurries.
Compounding coal's form handicap in the energy market are its impurities. A variable content of ash inflates shipping, emissions control, maintenance and disposal costs. Moisture is costly to ship and lowers boiler efficiencies. Sulfur and nitrogen contents are considered responsible for acid rain and require expensive control devices to meet clean air standards. Chlorine in some coals causes expensive boiler tube corrosion, and/or requires costly alloy. Moreover, chlorine is implicated in some of a long list of, as yet unregulated, trace toxics believed present in air emissions from coal burning (the most notorious being dioxins). The National Committee for Geochemistry of the National Research Council identified in 1980 the elements: arsenic, boron, cadmium, lead, mercury, molybdenum and selenium as "of greatest environmental concern" with respect to coal, and the elements: vanadium, chromium, nickel, copper, zinc and fluorine "of moderate concern".
The Illinois Basin is a major, well-located source of bituminous coal. While sulfur and chlorine contents vary from seam-to-seam, both tend to be relatively high. As environmental pressures have increased, the extensive mining industry in the area has suffered loss of business and jobs. A large research budget administered by the Illinois Clean Coal Institute (ICCI), formerly Center for Research on Sulfur in Coal (CRSC), has not produced an economical remedy for these impurities. My U.S. Pat. Nos. 4,714,032 and 5,050,375 specifically target the environmentally sound utilization of Illinois No. 6 coal with respect to sulfur and nitrogen oxides. The invention of this application addresses the removal of other impurities, particularly chlorine.
Conversion of MSW to energy is also impeded by impurities. Water contents are high and extremely variable. Ash contents are also high. About half of the 1000 metric tons of mercury used annually in the U.S. goes into disposable batteries, most of which wind up in MSW. Discarded batteries are also significant sources of cadmium and lead. Toxic lead, cadmium and mercury appear in the flue gas and have to be controlled by scrubbers and filters. Chlorine, originating with chlorinated plastics such as PVC (40% chlorine), averages about 0.5% but can range to as high as 1.8%. It engenders corrosive conditions in the firebox, requires alkali scrubbers and contributes to the formation of dioxins, furans and probably other dangerous air pollutants. Environmentalists are particularly sensitive to mercury vapor in flue gas; there is controvery over whether its methylated form is adequately accounted for.
In addition, bottom ash is sometimes classified as hazardous on the basis of the Environmental Protection Agency (EPA) TCLP leaching test and fly ash almost always is. Toxic metals which find their way into the ash include lead, arsenic, cadmium, selenium, chromium and mercury. Literature discloses that hazardous ash can be made to pass the TCLP test by heating it to or above its melting point, a process known as "vitrification".
Since, with coal burning, emissions of sulfur oxides are frequently a problem while MSW and fuel derived from it (RDF) are low in sulfur, there have been numerous attempts to blend the fuels so that flue gas from burning the blend (a practice known as co-firing) complies with sulfur oxide regulations. The sulfur advantage of co-firing has been overshadowed, however, by practical boiler problems, including higher requirement for excess air, poorer combustion control, increased slagging and corrosion. More recently there has been a revival of interest because of some evidence that sulfur oxides from the coal inhibits the formation of dioxins from chlorine in the RDF. However, RDF produced by conventional dry RR cannot be pulverized so as to be co-fired with pulverized coal, restricting the practice to the generally older and smaller stoker and moving grate boilers.
The world has thousands of sites at which garbage has been dumped for decades, and even centuries. Although dumping practice has improved and come to be known as "landfilling", much discarded refuse lies decaying under conditions now considered environmentally unsatisfactory. There is public pressure to remedy such old dump and landfill sites, which is bound to increase. Besides the potential hazards, much recyclable material and potential energy lies buried, awaiting economic means of recovery.
Many of the papermill sludges (besides being penalized by high water and ash contents) also contain serious amounts of chlorine and thus share the corrosion and air toxics risks hampering recovery of energy from MSW. Another potential biomass fuel high in chlorine is manure.
Although chlorine receives the lion's share of attention, MSW (as well as Industrial and Construction and Demolition Wastes) may contain lesser amounts of other halogens. In particular, fluorine from fluorinated (or chlorofluorinated) polymers may, in some cases, have toxic and/or fouling consequences. Both fluorine and bromine occur in some coals.
Environmentalists have long promoted tree farms, or other biomass crops, as renewable energy resources (which have the virtue of "recycling" carbon dioxide rather than adding to the production of greenhouse gases). The predominant impurity in renewable biomass fuels is water, which seriously detracts from, or even nullifies, their net energy value in atmospheric boilers. They generally have a low ash content which, nevertheless, can cause serious problems with low melting slags, frequently associated with sodium and/or potassium.